#include <bits/stdc++.h>
using namespace std;

int main() {
	int k;
	int sum = 0;
	int day = 1, money = 1;
	cin >> k;
	for (int i = 0; i < k; ++i) {
		sum += money;
		day--;
		if (day == 0) {
			money += 1;
			day = money;
		}
	}
	cout << sum;
	return 0;
}